Many modern technologies rely on the modification of surface material properties by physical and chemical processes to achieve particular surface characteristics. Some of these modification methods are applied broadly over an entire surface, such as the nitridation hardening of metal parts for use as drill bits or automotive engine valves. In these cases, it is important to know the variation of the modification from the surface into the bulk of the material along the vertical dimension. Another example, shown in FIG. 1, is the formation of an electrical junction in a silicon wafer by ion implantation and rapid thermal annealing. Modification methods can also be applied in a localized manner, such as the ion implantation of semiconductor materials through a sub-micron-scale patterned mask to produce localized circuit elements or “features” (see FIG. 2). The ion-implanted wafer is subsequently annealed to activate the dopant. In the localized cases, it is important to know the variation of the modification from the surface into the bulk of the material along the vertical dimension, and also to know the lateral variation of the modification across the transition from masked (not implanted) to non-masked (implanted) locations. The modification variation along the vertical (z) and lateral (x,y) dimensions can be termed the modification profile M(x,y,z). The total modification per unit area M0(x,y) produced in the wafer can be obtained by numerical integration over z. In the case of ion implantation, M0(x,y) is the total damage from the ion implantation and can be related by calibration to the dose of implanted ions.
For integrated circuit (IC) fabrication, localized regions of different resistivity are created in a silicon chip (wafer) by first creating a pattern or mask using well-known photolithography methods (FIG. 2). The mask material may be photoresist or SiO2, Si3N4, or any other material that will block the passage of the dopant. The modifier elements are then caused to enter the non-masked regions by various means such as ion implantation, plasma immersion or diffusion from an applied surface film. If the modifier entry causes disruption of the silicon crystal structure (lattice damage), as in the case of ion implantation, then it is possible to measure the amount and profile of the damage as an indirect measure of the modifier itself. If damage is not caused, or in any case after the wafer is annealed to remove lattice damage and electrically activate the modifier material, then it is possible to measure the amount and profile of the activated dopant material, D(x,y,z). This quantity is directly related to the electrical resistivity of the material which is very important technologically because it is a key parameter in the operation of the IC. This quantity also contains the “critical dimension” information pertaining to the ion implantation and annealing steps. An example is the important parameter, known as “junction depth” (zj), defined as the depth at which the activated dopant concentration falls off to a certain specified value (such as 1018/cm3) that roughly equals the concentration of free carriers (of the opposite type) in the non-implanted regions of the wafer. More generally, xj and yj also similarly denote the lateral fall off of the dopant between doped and non-doped local regions.
There is considerable interest in analyzing the quality of the implant in a non-destructive optical manner. One such device that is commercially available to analyze implants is marketed by the assignee herein under the Therma-Probe trademark. This device includes an intensity modulated pump beam which is focused onto the sample surface. The pump beam creates both periodic heating and generates a periodic plasma which diffuses through the sample. The diffusion characteristics are directly effected by the damage to the lattice structure caused by the implantation step. The diffusion of the thermal and plasma components is monitored with a separate probe beam which is focused within the periodically excited area on the wafer. The probe beam monitors the periodic changes in reflectivity which are induced by the modulated thermal and plasma waves. Details of the basic physics and operation of a Therma-Probe device can be found in the following patents, each of which is incorporated by reference: U.S. Pat. Nos. 4,579,463; 4,636,088; 4,854,710 and 5,978,074.
The Therma-Probe device directly detects damage to the lattice structure of the crystalline structure of the wafer which then must be correlated with dose. It is believed that the subject scatterometry approach described herein may be able to analyze both the depth and lateral dimensions of an implant directly without correlation. The Therma-Probe is also generally limited to measurements in non-patterned regions of the wafer, while the subject scatterometry approach may be able to operate in a patterned areas on a die.
Other non-contact optical metrology tools have been used to measure compositional characteristics of semiconductor wafers. Ellipsometry and reflectometry are two examples of commonly used optical techniques. A reflectometer measures the change in intensity of a probe beam that reflects from the surface of the sample. An ellipsometer monitors the change in polarization state of a probe beam induced by interaction with the sample. Historically, these devices have been used to monitor compositional characteristics such as layer thickness, index of refraction and extinction coefficient. Examples of such tools are discussed in the following patents which are incorporated herein by reference: U.S. Pat. Nos. 5,608,526 and 5,798,837.
More recently, ellipsometric and reflectometric measurements have been used in the field of scatterometry. Scatterometry is a radiation metrology method for determining the shape and properties of a physical object that “scatters” the radiation. Here, “radiation” includes any wavelength (λ) of the electromagnetic spectrum. Generally the wavelengths used for a particular application are those that span the dimensions of the features or localized variations of the sample (wafer). For modern IC production and advanced development, the key circuit feature lateral dimension (x0) range from 2000 nm (or 2×10−6 meters) down to less than 50 nm. Ideally the wavelength range might extend from 10 x0 to 1/10 x0, but the practical reality is that a more limited wavelength range can perform successfully. Today the scatterometry wavelength range typically used is 800 nm down to the shortest wavelength conveniently available, which is about 190 nm.
The term “scatter”, as in scatterometry, is a physics term meaning “to alter the propagation of a radiation wave”. In general, the alteration may include refraction, reflection and diffraction. Each material causing the scatter may also have a nonzero part of the wavelength-dependent, complex refractive index ñ(λ), leading to radiation absorption in addition to the above non-dissipative effects. Diffraction is present in all radiation propagation and scattering. However, to the extent that x0 is less than the radiation wavelengths being employed in the scatterometry, then diffraction will play a stronger role in the scattering and the physical modeling must treat the diffraction component with increasing detail and accuracy for success.
Scatterometry generally employs an array of repetitive features in order to maximize the diffractive (constructive interference) component of the scattering. Simple line/space features (FIG. 2) are commonly employed currently. While most of the scatterometry studies have been directed to periodic line structures, these efforts have been extended to more complex 3D structures (e.g. vias) and even to isolated or aperiodic structures. The extension to non-periodic structures may require more complex mathematical modeling and a greater effort to maintain adequate signal-to-noise ratio in the apparatus but is nonetheless contemplated by this disclosure.
The basic diagram of a scatterometry system is shown in FIGS. 3a to 3c. Scatterometry may employ any of the following: an ellipsometric setup; a reflectometric setup; a range of incident wavelengths at one angle of incidence; one wavelength over a range of angles of incidence; both a range of wavelengths and a range of angles of incidence; or simply one wavelength and a single angle of incidence. These different setups result in different amounts of information content encoded onto the scattered light beam, resulting in different quality of detail and resolution in the desired extracted parameters. The preferred method that acquires the most information content, yet is practical for manufacturing use, is the broad-wavelength spectroscopic ellipsometer method, of rotating compensator design, at one angle of incidence. (See for example, U.S. Pat. No. 5,877,859, incorporated herein by reference.)
However, the application method disclosed here is independent of the details of the scatterometer apparatus design.
Additional background information on scatterometers and scatterometry analysis can be found in the following publications which are incorporated herein by reference. U.S. Pat. No. 5,607,800 (Ziger); U.S. Pat. No. 5,867,276 (McNeil); U.S. Pat. No. 5,739,909 (Blayo); U.S. Pat. No. 6,429,943 (Opsal); U.S. Pat. No. 6,483,580 (Xu); U.S. Pat. No. 2002/0038196 (Johnson) and U.S. Pat. No. 2002/0035455 (Niu).
As can be seen from the above citations, scatterometry has been proven capable of determining the cross-sectional shape of the line/space element of a repetitive array (grating) of IC features (such as the mask line/space in FIG. 2) which sit upon a substrate (such as the Si in FIG. 2). This technique has been applied to several IC processing steps including the photoresist develop-inspect step in lithography processing and the final-inspect step of etch processing.
It has recently been proposed that scatterometry system could be used to or monitor the ion implantation process. More specifically, U.S. Pat. No. 6,451,621, ('621 patent) incorporated herein by reference, notes that the implantation of dopants is controlled by an initial application of a photolithographic mask. As noted above, the mask often has grating-like properties in the active area of the wafer. Alternatively, the mask is often designed to provide test gratings in the inactive regions (scribe lines) of the wafer. The '621 patent proposes to measure the gratings of the mask using scatterometry. Since the mask is used to control the implant, accurate measurements of the mask can provide information about the dopants that will be implanted during the implantation step. It should be noted that the '621 patent only discloses measuring the mask prior to the implantation step as way to control or predict the implantation process. The '621 patent does not disclose using scatterometry to actually measure the dopants after implantation.
U.S. Pat. No. 5,963,329 ('329 patent) also relates to scatterometry techniques for periodic structures. The '329 patent suggests that it might be possible to measure doping profiles using scatterometry. However, the '329 patent discloses a relatively complex and undesirable approach for carrying out the measurement. More specifically, the '329 patent suggests that it would be necessary to etch a grating structure into the wafer after the implantation step. The scatterometry tool would be used to measure the variations in the sidewalls of the grating in order to determine doping profiles. As can be appreciated, this approach requires an additional processing step in the wafer manufacture that would essentially destroy the wafer.